Semi.Avalonia/demo/Semi.Avalonia.Demo/Pages/TreeViewDemo.axaml.cs

using System.Collections.ObjectModel;
using Avalonia.Controls;
using CommunityToolkit.Mvvm.ComponentModel;

namespace Semi.Avalonia.Demo.Pages;

public partial class TreeViewDemo : UserControl
{
    public TreeViewDemo()
    {
        InitializeComponent();
        this.DataContext = new TreeViewVm();
    }
}

public class TreeViewVm : ObservableObject
{
    public ObservableCollection<TreeViewItemVm> Items { get; set; }

    public ObservableCollection<FirstItem>? MultipleLevelItems { get; set; }

    public TreeViewVm()
    {
        Items =
        [
            new TreeViewItemVm { Name = "Item 1", Id = "1" },
            new TreeViewItemVm { Name = "Item 2", Id = "2" },
            new TreeViewItemVm
            {
                Name = "Item 3", Id = "3", Items =
                [
                    new TreeViewItemVm { Name = "Item 3.1", Id = "3.1" },
                    new TreeViewItemVm { Name = "Item 3.2", Id = "3.2" },
                    new TreeViewItemVm { Name = "Item 3.3", Id = "3.3" }
                ],
            }

        ];

        MultipleLevelItems = [];
        for (var i = 1; i < 6; i++)
        {
            var firstItem = new FirstItem
            {
                Id = i, Name = $"FirstItem {i}",
                SecondItems = []
            };
            for (var j = 1; j < 6; j++)
            {
                var secondItem = new SecondItem
                {
                    Id = j, Name = $"SecondItem {j}",
                    ThirdItemItems = []
                };
                for (var k = 1; k < 6; k++)
                {
                    var thirdItem = new ThirdItem { Id = k, Name = $"ThirdItem {k}" };
                    secondItem.ThirdItemItems.Add(thirdItem);
                }

                firstItem.SecondItems.Add(secondItem);
            }

            MultipleLevelItems.Add(firstItem);
        }
    }
}

public partial class TreeViewItemVm : ObservableObject
{
    public ObservableCollection<TreeViewItemVm> Items { get; set; } = [];
    public string? Name { get; set; }
    public string? Id { get; set; }
}

public class ItemBase
{
    public int Id { get; set; }
    public string? Name { get; set; }
}

public class FirstItem : ItemBase
{
    public ObservableCollection<SecondItem>? SecondItems { get; set; }
}

public class SecondItem : ItemBase
{
    public ObservableCollection<ThirdItem>? ThirdItemItems { get; set; }
}

public class ThirdItem : ItemBase
{
}